libvpx

Enable adaptive pred filter type for sub8x8

This commit enables an adaptive prediction filter type selection
for sub8x8 block sizes. In speed 1, it re-uses the filter type of
collocated 8x8 block if it is tested in the rate-distortion optimization
loop, for the sub8x8 blocks. Otherwise, it runs the normal test
over all the three filter types. In speed 2, it re-uses the 8x8
block's prediction filter type, if available. Otherwise, force it
to be EIGHTTAP.

Compression and speed performance wise:
speed 1
derf -0.266%
yt   -0.138%

bus at 2000 kbps: 33766ms -> 30451ms (10% speed-up)
football at 600 kbps: 48173ms -> 43786ms (9% speed-up)

speed 2
derf -0.026%
yt   +0.134%

bus at 2000 kbps: 18973ms -> 17698ms (6% speed-up)
football at 600 kbps: 26748ms -> 25096ms (6% speed-up)

Change-Id: I77e097533b969fd3472147225fa79fc98095d342